Really fix typo in docs.

g3doc/design.md

diff --git a/g3doc/design.md b/g3doc/design.md
index 9c6093e..afa3ea1 100644
--- a/g3doc/design.md
+++ b/g3doc/design.md
@@ -10,7 +10,7 @@
 stable intermediate representation (IR), which is consumed by the back ends.
 This IR is defined in [public/ir_pb2.py][ir_pb2_py].
 
-[ir_pb_py]: public/ir_pb2.py
+[ir_pb2_py]: public/ir_pb2.py
 
 The back ends read the IR and emit code to view and manipulate Emboss-defined
 data structures.  Currently, only a C++ back-end exists.