fix some spelling in man page

sent.1

@@ -9,8 +9,8 @@
 .Op Ar file
 .Sh DESCRIPTION
 .Nm
-is a simple plaintext presentation tool for X. sent does not need LaTeX,
-LibreOffice or any other fancy file format. Instead, sent reads plaintext
+is a simple plain text presentation tool for X. sent does not need LaTeX,
+LibreOffice or any other fancy file format. Instead, sent reads plain text
 describing the slides. sent can also draw images.
 .Pp
 Every paragraph represents a slide in the presentation. Especially for
@@ -27,18 +27,18 @@ Print version information to stdout and exit.
 .It Em Mouse commands
 .Bl -tag -width Ds
 .It Sy Button1 | Button5
-Go to next slide, if existant.
+Go to next slide, if existent.
 .It Sy Button3 | Button4
-Go to previous slide, if existant.
+Go to previous slide, if existent.
 .El
 .It Em Keyboard commands
 .Bl -tag -width Ds
 .It Sy Escape | q
 Quit.
 .It Sy Right | Return | Space | l | j | Down | Next | n
-Go to next slide, if existant.
+Go to next slide, if existent.
 .It Sy Left | Backspace | h | k | Up | Prior | p
-Go to previous slide, if existant.
+Go to previous slide, if existent.
 .El
 .El
 .Sh FORMAT