README: update number of contributors

Change-Id: I5a1c7ed83a430a509d8f61f4aba8772d5d16ad48
GitHub-Last-Rev: fed86d88691c8a62bafac18a4e5c0944a4e59050
GitHub-Pull-Request: golang/go#24664
Reviewed-on: https://go-review.googlesource.com/104515Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

README.md

@@ -31,7 +31,7 @@ in your web browser for source installation instructions.
 
 ### Contributing
 
-Go is the work of hundreds of contributors. We appreciate your help!
+Go is the work of thousands of contributors. We appreciate your help!
 
 To contribute, please read the contribution guidelines:
 	https://golang.org/doc/contribute.html