Fix unterminated double-backtick in a git.compat docstring

Adding git.compat in reference.rst revealed this (and makes this
fix necessary for the documentation to build).

git/compat.py

@@ -61,7 +61,7 @@
 
 :note:
     For macOS (Darwin), ``os.name == "posix"`` as in other Unix-like systems, while
-    ``sys.platform == "darwin"`.
+    ``sys.platform == "darwin"``.
 """
 
 defenc = sys.getfilesystemencoding()