cleanup: misc typos in strings and comments

Found via `codespell`

Closes #2389

docs/INSTALL.cmake

@@ -34,7 +34,7 @@ Current flaws in the curl CMake build
    - Doesn't allow you to disable specific protocols from the build
    - Doesn't find or use krb4 or GSS
    - Rebuilds test files too eagerly, but still can't run the tests
-   - Does't detect the correct strerror_r flavor when cross-compiling (issue #1123)
+   - Doesn't detect the correct strerror_r flavor when cross-compiling (issue #1123)
 
 
 Command Line CMake

docs/TODO

@@ -710,7 +710,7 @@ that doesn't exist on the server, just like --ftp-create-dirs.
  You can specify the private and public keys for SSH/SSL as file paths. Some
  programs want to avoid using files and instead just pass them as in-memory
  data blobs. There's probably a challenge to make this work across the
- plethory of different TLS and SSH backends that curl suppports.
+ plethory of different TLS and SSH backends that curl supports.
  https://github.com/curl/curl/issues/2310
 
 13.4 Cache/share OpenSSL contexts

docs/examples/curlx.c

@@ -183,7 +183,7 @@ static unsigned char *my_get_ext(X509 *cert, const int type,
 
 /* This is an application verification call back, it does not
    perform any addition verification but tries to find a URL
-   in the presented certificat. If found, this will become
+   in the presented certificate. If found, this will become
    the URL to be used in the POST.
 */
 

docs/examples/shared-connection-cache.c

@@ -20,7 +20,7 @@
  *
  ***************************************************************************/
 /* <DESC>
- * Connection cache shared between easy handles with the share inteface
+ * Connection cache shared between easy handles with the share interface
  * </DESC>
  */
 #include <stdio.h>