Fix a couple of trivial typos (GH-4363)

See
https://github.com/cython/cython/pull/4349
https://github.com/cython/cython/commit/0574dbceef7b8ee16a9cc94091c3629dfa23133d#r54025763

docs/src/userguide/language_basics.rst

@@ -1286,7 +1286,7 @@ constants within a Cython source file.
 
 .. note::
 
-    This feature has very little use cases.  Specifically, is is not a good
+    This feature has very little use cases.  Specifically, it is not a good
     way to adapt code to platform and environment.  Use code generation or
     (preferably) C compile time adaptation for this.  See, for example,
     :ref:`verbatim_c`.

tests/errors/cpp_bool.pyx

@@ -5,7 +5,7 @@ from libcpp.string cimport string
 
 cdef foo():
     cdef string field
-    if field:  # field cannot be coerced to book
+    if field:  # field cannot be coerced to bool
         pass
 
 _ERRORS = u"""