Fix typo. Fixes #112.

c++/src/capnp/compiler/capnp.c++

@@ -195,7 +195,7 @@ public:
           "    capnp eval myschema.capnp someConstant.someList[4]\n"
           "    capnp eval myschema.capnp someConstant.someList[4].anotherField[1][2][3]\n"
           "Since consts can have complex struct types, and since you can define a const using "
-          "imporst and variable substitution, this can be a convenient way to write text-format "
+          "import and variable substitution, this can be a convenient way to write text-format "
           "config files which are compiled to binary before deployment.",
 
           "By default the value is written in text format and can have any type.  The -b, -p, "