standardize format of keyboard shortcuts (#1354)

* fix capitalization of keyboard shortcuts (elinks, tmux, screen)
* adjust formatting of keyboard shortcuts (tail, xsel)

pages/common/elinks.md

@@ -8,7 +8,7 @@
 
 - Quit elinks:
 
-`ctrl+c`
+`Ctrl+C`
 
 - Dump output of webpage to console, colorizing the text with ANSI control codes:
 

pages/common/screen.md

@@ -24,7 +24,7 @@
 
 - Detach from inside a screen:
 
-`ctrl+A D`
+`Ctrl+A D`
 
 - Kill a detached screen:
 

pages/common/tail.md

@@ -14,6 +14,6 @@
 
 `tail -c {{num}} {{file}}`
 
-- Keep reading file until ctrl-c:
+- Keep reading file until `Ctrl+C`:
 
 `tail -f {{file}}`

pages/common/tmux.md

@@ -24,7 +24,7 @@
 
 - Detach from session:
 
-`ctrl+b d`
+`Ctrl+B D`
 
 - Kill session: