README: Fix small typo, dfeault -> default.

README.md

@@ -144,7 +144,7 @@ options (like cross-compiling), the same set of options should be passed
 to `make deplibs`. To actually enable/disable use of dependencies, edit
 `ports/unix/mpconfigport.mk` file, which has inline descriptions of the options.
 For example, to build SSL module (required for `upip` tool described above,
-and so enabled by dfeault), `MICROPY_PY_USSL` should be set to 1.
+and so enabled by default), `MICROPY_PY_USSL` should be set to 1.
 
 For some ports, building required dependences is transparent, and happens
 automatically.  But they still need to be fetched with the `make submodules`