[TNTP-931] Fix vshard.router.bootstrap example in README

The example with vshard.router.bootstrap polling has been removed. Instead, the following call is now used: vshard.router.bootstrap({timeout = TIMEOUT, if_not_bootstrapped = true}).

This approach aligns with the vshard documentation vshard.router.bootstrap and prevents the error: \['UNKNOWN_BUCKETS', '15000 buckets are not discovered'\].

(cherry picked from commit 8c1813eaf9bb26aa371b1034ecc586a70cd722fd)

Author: Satbek

README.md

@@ -2034,28 +2034,12 @@ issues.
 
     ```lua
     -- myrouter.lua
-
-    local clock = require('clock')
-    local fiber = require('fiber')
     local log = require('log')
-
     local vshard = require('vshard')
 
-    local TIMEOUT = 60
-    local DELAY = 0.1
-
-    local start = clock.monotonic()
-    while clock.monotonic() - start < TIMEOUT do
-        local ok, err = vshard.router.bootstrap({
-            if_not_bootstrapped = true,
-        })
-
-        if ok then
-            break
-        end
-
-        log.info(('Router bootstrap error: %s'):format(err))
-        fiber.sleep(DELAY)
+    local ok, err = vshard.router.bootstrap({timeout = 15})
+    if not ok then
+      log.info(('Router bootstrap error: %s'):format(err))
     end
     ```