_content/blog/intro-generics: s/generics is/generics are/

Change-Id: I9399ec4d0ce4c5ef5a346b51a11610c743cba091
Reviewed-on: https://go-review.googlesource.com/c/website/+/395614
Reviewed-by: Robert Griesemer <gri@google.com>
Trust: Ian Lance Taylor <iant@google.com>
diff --git a/_content/blog/intro-generics.md b/_content/blog/intro-generics.md
index 15e48f7..160b5ad 100644
--- a/_content/blog/intro-generics.md
+++ b/_content/blog/intro-generics.md
@@ -31,7 +31,7 @@
 what the proposal document permits; the spec should be accurate.
 Future releases may lift some of the restrictions.)
 
-Generics is a way of writing code that is independent of the specific
+Generics are a way of writing code that is independent of the specific
 types being used.
 Functions and types may now be written to use any of a set of types.