docs: fix links, grammar and spelling in Markdown

doc/guides/compile.md

@@ -265,7 +265,7 @@ There is a `RubyGem` (gem for CRuby) named `mgem` that help you to
 manage `mrbgems`. Try `gem install mgem`. `mgem` can show you the list
 of registered `mrbgems`.
 
-See doc/mrbgems/README.md for more option about mrbgems.
+See [doc/guides/mrbgems.md](mrbgems.md) for more option about mrbgems.
 
 ### Mrbtest
 

doc/guides/gc-arena-howto.md

@@ -62,7 +62,7 @@ memory leak.
 
 As of this writing, mruby automatically extend arena to remember
 objects (See `MRB_GC_FIXED_ARENA` and `MRB_GC_ARENA_SIZE` in
-doc/guides/mrbconf.md).
+[doc/guides/mrbconf.md](mrbconf.md)).
 
 If you create many objects in C functions, memory usage will increase, since
 GC never kicks in. This memory usage may look like memory leaks, but will also

doc/guides/mrbgems.md

@@ -1,6 +1,6 @@
 # mrbgems
 
-mrbgems is a library manager to integrate C and Ruby extension in an easy and
+mrbgems is a library manager to integrate C and Ruby extensions in an easy and
 standardised way into mruby. Conventionally, each mrbgem name is prefixed by
 `mruby-`, e.g. `mruby-time` for a gem that provides `Time` class functionality.
 

doc/mruby3.0.md

@@ -159,4 +159,4 @@ For better and faster random number generation.
 
 Preallocated symbols are interned at compile-time. They can be accessed via symbols macros (e.g. `MRB_SYM()`).
 
-See [Symbols](https://github.com/mruby/mruby/blob/master/doc/guides/symbol.md).
+See [Symbols](guides/symbol.md).