Fix GitHub repository indications for crypto changes in 2.20

The content was originally written for mbed-crypto. Change pull
request references to be relative to mbedtls instead.
diff --git a/ChangeLog b/ChangeLog
index 2335eab..4a043cc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -25,7 +25,7 @@
      to achieve the security strength defined by NIST SP 800-90A. You can
      change it with mbedtls_ctr_drbg_set_nonce_len().
    * Add ENUMERATED tag support to the ASN.1 module. Contributed by
-     msopiha-linaro in #307.
+     msopiha-linaro in ARMmbed/mbed-crypto#307.
 
 API changes
    * In the PSA API, forbid zero-length keys. To pass a zero-length input to a
@@ -42,7 +42,7 @@
    * Remove a useless assignment. Reported and fix submitted by irwir.
      Fixes #2801.
    * Fix a buffer overflow in the PSA HMAC code when using a long key with an
-     unsupported algorithm. Fixes #254.
+     unsupported algorithm. Fixes ARMmbed/mbed-crypto#254.
    * Fix mbedtls_asn1_get_int to support any number of leading zeros. Credit
      to OSS-Fuzz for finding a bug in an intermediate version of the fix.
    * Fix mbedtls_asn1_get_bitstring_null to correctly parse bitstrings of at
@@ -51,7 +51,7 @@
      mbedtls_hmac_drbg_set_entropy_len() now work if you call them before
      mbedtls_ctr_drbg_seed() or mbedtls_hmac_drbg_seed().
    * Fix some false-positive uninitialized variable warnings. Fix contributed
-     by apple-ihack-geek in ARMmbed/mbedtls#2663.
+     by apple-ihack-geek in #2663.
 
 Changes
    * Remove the technical possibility to define custom mbedtls_md_info
@@ -62,13 +62,13 @@
      rather than success, so that coding mistakes or memory corruption tends to
      cause functions to return this error code rather than a success. There are
      no known instances where this changes the behavior of the library: this is
-     merely a robustness improvement. #323
+     merely a robustness improvement. ARMmbed/mbed-crypto#323
    * Remove a useless call to mbedtls_ecp_group_free(). Contributed by
-     Alexander Krizhanovsky in #210.
+     Alexander Krizhanovsky in ARMmbed/mbed-crypto#210.
    * Speed up PBKDF2 by caching the digest calculation. Contributed by Jack
-     Lloyd and Fortanix Inc in #277.
+     Lloyd and Fortanix Inc in ARMmbed/mbed-crypto#277.
    * Small performance improvement of mbedtls_mpi_div_mpi(). Contributed by
-     Alexander Krizhanovsky in #308.
+     Alexander Krizhanovsky in ARMmbed/mbed-crypto#308.
 
 = mbed TLS 2.19.1 branch released 2019-09-16