[rom_ext] Address Typos in Usage/Comment

Signed-off-by: Sam Elliott <selliott@lowrisc.org>
diff --git a/util/rom-ext-manifest-generator.py b/util/rom-ext-manifest-generator.py
index 525907b..3256770 100755
--- a/util/rom-ext-manifest-generator.py
+++ b/util/rom-ext-manifest-generator.py
@@ -18,7 +18,7 @@
     Directory where manifest.hjson and manifest.h.tpl reside.
 
   rom-ext-manifest-generator --output-dir:
-    Directory where manifest.hjson and manifest.h.tpl reside.
+    Directory where manifest.h will be created.
 """
 
 
@@ -26,7 +26,7 @@
     """ Generates C header file from the `template_file`.
 
     It produces a list of tuples with a field name and the `MemoryRegion`
-    object, whic is used in the `template_path`. The resulting header file is
+    object, which is used in the `template_path`. The resulting header file is
     placed into `output_path`.
     """