spec: fix link for instantiations

This change corrects the link `Instantiantions` to `Instantiations` in the spec.

Change-Id: Ib0ed03420ae401d20af1ea723c5487018b2f462d
GitHub-Last-Rev: b84316c
GitHub-Pull-Request: golang#49816
Reviewed-on: https://go-review.googlesource.com/c/go/+/367274
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>

Author: syumai

doc/go_spec.html

@@ -2883,7 +2883,7 @@ <h3 id="Method_declarations">Method declarations</h3>
 
 <p>
 Syntactically, this type parameter declaration looks like an
-<a href="#Instantiantions">instantiation</a> of the receiver base type, except that
+<a href="#Instantiations">instantiation</a> of the receiver base type, except that
 the type arguments are the type parameters being declared, one for each type parameter
 of the receiver base type.
 The type parameter names do not need to match their corresponding parameter names in the