Fix invalid characters for Common Lisp symbols

The characters [ ] { and } are perfectly fine to use in variable names.

(defparameter b{l}a[b[l]]a "test") ; => B{L}A[B[L]]A
b{l}a[b[l]]a                       ; => "test"
Fix typo in Common Lisp introduction.
Merge pull request #590 from jorams/cl-intro-typo

[common-lisp/en] Fix typo in Common Lisp introduction.