docs: Fix spelling mistakes

Change-Id: I8a68156d785a260fa334766337d4626ff28aec6a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39920
Pigweed-Auto-Submit: Ali Zhang <alizhang@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/docs/module_structure.rst b/docs/module_structure.rst
index 7334b53..17d3a07 100644
--- a/docs/module_structure.rst
+++ b/docs/module_structure.rst
@@ -375,7 +375,7 @@
 .. caution::
 
   Modules should only use facades when necessary. Facades are permanently locked
-  to a particular implementation at compile time. Multpile backends cannot be
+  to a particular implementation at compile time. Multiple backends cannot be
   used in one build, and runtime dependency injection is not possible, which
   makes testing difficult. Where appropriate, modules should use other
   mechanisms, such as virtual interfaces, callbacks, or templates, in place of