[roman-numerals] Tidy up grammer and formatting. (#2368)

[roman-numerals] Tidy up grammer and formatting.

Author: IsaacG

exercises/roman-numerals/description.md

@@ -9,8 +9,7 @@ One thing they never discovered though was the number zero.
 This made writing and dating extensive histories of their exploits slightly more challenging, but the system of numbers they came up with is still in use today.
 For example the BBC uses Roman numerals to date their programs.
 
-The Romans wrote numbers using letters - I, V, X, L, C, D, M.
-(notice these letters have lots of straight lines and are hence easy to hack into stone tablets).
+The Romans wrote numbers using letters - I, V, X, L, C, D, M; notice these letters have lots of straight lines and are hence easy to hack into stone tablets.
 
 ```text
  1  => I
@@ -19,22 +18,29 @@ The Romans wrote numbers using letters - I, V, X, L, C, D, M.
 ```
 
 The maximum number supported by this notation is 3,999.
-(The Romans themselves didn't tend to go any higher)
 
-Wikipedia says: Modern Roman numerals ... are written by expressing each digit separately starting with the left most digit and skipping any digit with a value of zero.
+Wikipedia says: Modern Roman numerals [...] are written by expressing each digit separately starting with the left most digit and skipping any digit with a value of zero.
 
 To see this in practice, consider the example of 1990.
 
 In Roman numerals 1990 is MCMXC:
 
-1000=M
-900=CM
-90=XC
+```text
+ 1000 => M
+  900 => CM
++  90 => XC
+----- => -----
+ 1990 => MCMXC
+```
 
 2008 is written as MMVIII:
 
-2000=MM
-8=VIII
+```text
+ 2000 => MM
++   8 => VIII
+----- => ------
+ 2008 => MMVIII
+```
 
 Learn more about [Roman numerals on Wikipedia][roman-numerals].