Fix a typo in README.md

This fixes a typo in README.md.

no-tn-check

Change-Id: Ia85e59009710859e942cc812c4ef60ad820f17e0

Author: tromey

README.md

@@ -68,7 +68,7 @@ basis: see the `info/enable/disable pretty-printer` commands in [GDB's
 manual](https://sourceware.org/gdb/onlinedocs/gdb/Pretty_002dPrinter-Commands.html).
 
 Note that these helpers are intrinsically tied to specific runtime
-implemenations. For instance, when the implementation of a container is
+implementations. For instance, when the implementation of a container is
 changed, the corresponding pretty-printer must be updated. For now, the master
 branch of this repository is synchronized with GNAT Pro's development branch.