go/doc: remove alias => operator from spec

[bradleyfalzon]

What version of Go are you using (go version)?

master @ d24b57a

What operating system and processor architecture are you using (go env)?

Not applicable.

What did you do?

Opened https://tip.golang.org/ref/spec#Operators_and_Delimiters

What did you expect to see?

I do not expect to see => listed as an operator.

What did you see instead?

I saw => listed as an operator, since alias #16339 related commits have been reverted , I believe this may have been missed.

CL that created this: https://go-review.googlesource.com/#/c/32310/ in commit 7fd6b92.

[gopherbot]

CL https://golang.org/cl/33365 mentions this issue.