Fix heading levels

Author: ohsayan

docs/blueql/1.overview.md

@@ -45,34 +45,34 @@ keywords:
 
 ## Data types
 
-#### Boolean
+### Boolean
 A boolean value, either `true` or `false`
 
-#### Unsigned integers
+### Unsigned integers
 
 - `uint8`: unsigned 8-bit integer
 - `uint16`: unsigned 16-bit integer
 - `uint32`: unsigned 32-bit integer
 - `uint64`: unsigned 64-bit integer
 
-#### Signed integers
+### Signed integers
 
 - `sint8`: signed 8-bit integer
 - `sint16`: signed 16-bit integer
 - `sint32`: signed 32-bit integer
 - `sint64`: signed 64-bit integer
 
-#### Floating point values
+### Floating point values
 
 - `float32`: a single-precision float
 - `float64`: a double-precision float
 
-#### Simple collections
+### Simple collections
 
 - `binary`: a binary blob represented as a sequence of `uint8` values
 - `string`: an UTF-8 string
 
-#### Complex collections
+### Complex collections
 
 - `list`: a list of any of the data types, including nested lists
   - A list is represented as: `[]` with values inbetween. For example, a `list { type:string }` would be represented as: