Update README.md for 7.3.1 (#97)

Automated update of `README.md` for release 7.3.1

Co-authored-by: keith <keith@users.noreply.github.com>

README.md

diff --git a/README.md b/README.md
index 4c24036..1faf4b3 100644
--- a/README.md
+++ b/README.md
@@ -46,7 +46,7 @@
 ```python
 bazel_dep(
     name = "buildifier_prebuilt",
-    version = "7.1.2",
+    version = "7.3.1",
     dev_dependency = True,
 )
 ```
@@ -63,10 +63,10 @@
 
 http_archive(
     name = "buildifier_prebuilt",
-    sha256 = "481f220bee90024f4e63d3e516a5e708df9cd736170543ceab334064fa773f41",
-    strip_prefix = "buildifier-prebuilt-7.1.2",
+    sha256 = "7f85b688a4b558e2d9099340cfb510ba7179f829454fba842370bccffb67d6cc",
+    strip_prefix = "buildifier-prebuilt-7.3.1",
     urls = [
-        "http://github.com/keith/buildifier-prebuilt/archive/7.1.2.tar.gz",
+        "http://github.com/keith/buildifier-prebuilt/archive/7.3.1.tar.gz",
     ],
 )