Update version

manifest.chrome.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name"            : "Markdown Viewer",
-  "version"         : "5.0",
+  "version"         : "5.1",
   "description"     : "Markdown Viewer",
 
   "homepage_url": "https://chrome.google.com/webstore/detail/markdown-viewer/ckkdlimhmcjmikdlpkmbgfkaikojcbjk",

manifest.firefox.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name"            : "Markdown Viewer",
-  "version"         : "5.0",
+  "version"         : "5.1",
   "description"     : "Markdown Viewer",
 
   "homepage_url": "https://addons.mozilla.org/en-US/firefox/addon/markdown-viewer-chrome/",