update shell/README

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/README

@@ -34,7 +34,7 @@ Shell Command Language
 
 It says that shell must implement special built-ins. Special built-ins
 differ from regular ones by the fact that variable assignments
-done on special builtin is *PRESERVED*. That is,
+done on special builtin are *PRESERVED*. That is,
 
 VAR=VAL special_builtin; echo $VAR
 
@@ -43,7 +43,7 @@ should print VAL.
 (Another distinction is that an error in special built-in should
 abort the shell, but this is not such a critical difference,
 and moreover, at least bash's "set" does not follow this rule,
-which is even codified in autoconf now...).
+which is even codified in autoconf configure logic now...)
 
 List of special builtins:
 
@@ -73,7 +73,7 @@ unset [-fv] name...
 In practice, no one uses this obscure feature - none of these builtins
 gives any special reasons to play such dirty tricks.
 
-However. This section says that *function invocation* should act
+However. This section also says that *function invocation* should act
 similar to special built-in. That is, variable assignments
 done on function invocation should be preserved after function invocation.