docs: add missing @ on export attribute

doc/docs.md

@@ -7443,7 +7443,7 @@ By default all V functions have the following naming scheme in C: `[module name]
 
 For example, `fn foo() {}` in module `bar` will result in `bar__foo()`.
 
-To use a custom export name, use the `[export]` attribute:
+To use a custom export name, use the `@[export]` attribute:
 
 ```
 @[export: 'my_custom_c_name']

vlib/v/help/build/build-c.txt

@@ -68,17 +68,17 @@ see also `v help build`.
          bare_panic(msg string)
             Print "V panic: " + msg, along with an optional backtrace
             and/or the V commit hash, and then exit.
-         [export: 'malloc']
+         @[export: 'malloc']
          __malloc(n usize) &C.void
             Allocates n bytes of memory and returns the pointer to the first
             byte.
-         [export: 'free']
+         @[export: 'free']
          __free(ptr &C.void)
             Free the block of memory ptr allocated by malloc.
          realloc(old_area &C.void, new_size usize) &C.void
             Allocates a new area of size new_size, copies old_area
             to the new area, and returns a pointer to the new area.
-         [export: 'calloc']
+         @[export: 'calloc']
          __calloc(nmemb usize, size usize) &C.void
             Like malloc, but sets all the bytes to `0` first.
          memcpy(dest &C.void, src &C.void, n usize) &C.void
@@ -102,7 +102,7 @@ see also `v help build`.
          bare_backtrace() string
             Return a backtrace that can be printed. If backtraces are not
             supported, return a message stating that backtraces do not work.
-         [export: 'exit']
+         @[export: 'exit']
          __exit(code int)
             Exit with code code. code is allowed to be ignored.