Fix formatting and spelling lint

[kortschak]

This fixes a collection of spelling errors in user-facing and internal documentation and fixes a test-failing vet error (incorrect *testing.T method used), and fixes a style lint error.

There are also a bunch of non-fatal lint vet errors that are reasonably arguably false positive but which the Go team will not fix vet for (see golang/go#49350). I am happy to send fixes for those here as well if wanted.

# github.com/google/cel-go/codelab
codelab/codelab.go:60:2: fmt.Println arg list ends with redundant newline
codelab/codelab.go:70:2: fmt.Println arg list ends with redundant newline
codelab/codelab.go:85:2: fmt.Println arg list ends with redundant newline
codelab/codelab.go:95:2: fmt.Println arg list ends with redundant newline
codelab/codelab.go:104:2: fmt.Println arg list ends with redundant newline
codelab/codelab.go:115:2: fmt.Println arg list ends with redundant newline
codelab/codelab.go:126:2: fmt.Println arg list ends with redundant newline
codelab/codelab.go:140:2: fmt.Println arg list ends with redundant newline
# github.com/google/cel-go/codelab/solution
codelab/solution/codelab.go:60:2: fmt.Println arg list ends with redundant newline
codelab/solution/codelab.go:99:2: fmt.Println arg list ends with redundant newline
codelab/solution/codelab.go:132:2: fmt.Println arg list ends with redundant newline
codelab/solution/codelab.go:163:2: fmt.Println arg list ends with redundant newline
codelab/solution/codelab.go:219:2: fmt.Println arg list ends with redundant newline
codelab/solution/codelab.go:259:2: fmt.Println arg list ends with redundant newline
codelab/solution/codelab.go:328:2: fmt.Println arg list ends with redundant newline
codelab/solution/codelab.go:366:2: fmt.Println arg list ends with redundant newline

Please take a look.

[TristonianJones]

/gcbrun